Recently, non-volatile memories that are data-rewritable semiconductor devices have become widely used. Such non-volatile memories include flash memories having a SONOS (Silicon Oxide Nitride Oxide Silicon) structure that accumulates charges in a trapping layer in an ONO (Oxide/Nitride/Oxide) film.
U.S. Pat. No. 6,011,725 discloses a SONOS flash memory that has virtual-ground memory cells each performing an interchange between the source and the drain, and a method of operating the source and the drain in a symmetrical fashion. In this flash memory, bit lines that also serve as the source and the drain are formed in the semiconductor substrate, and charges can be accumulated in the trapping layer in an ONO film formed on the semiconductor substrate. By performing an interchange between the source and the drain, two charge accumulating regions can be formed in one memory cell.
In such a memory cell, data writing (or charge accumulation in the ONO film) is performed by applying a high voltage between the source and the drain, so as to generate positive charges at the gate. As a result, hot electrons generated with high energy between the source and the drain, are injected into the trapping layer. The source and the drain are interchanged with each other, so that charges can be stored at two locations in the trapping layer. Through the accumulation of charges (electrons) in the trapping layer, the threshold voltage of the transistor forming the memory cell also changes. By detecting the threshold voltage, the data can be read out.
Data erasing (or removal of charges from the trapping layer) in such a memory cell is performed by the following methods. By one of the methods, a positive voltage is applied to the drain, and a negative voltage is applied to the gate. As a result, electron-hole pairs are generated due to interband tunneling in the vicinity of the drain, and hot holes generated with high energy in the electric field between the gate and the drain are injected into the trapping layer. The holes injected into the trapping layer are coupled with the charges (electrons) accumulated in the trapping layer, and the charges in the trapping layer disappear. By another method, a negative voltage is applied to the gate, and a positive voltage is applied to the semiconductor substrate. As a result, the charges in the trapping layer are released as a FN (Fowler-Nordheim) tunneling current, and the charges disappear from the inside of the trapping layer.